The present invention generally relates to methods to adhesively bond sections of a Pi-joint design and, more particularly, to an adhesive injection process for Pi-joint assemblies.
Adhesive bonded primary aircraft structures have the potential to save significant weight and cost over conventional bonded and riveted or bolted joint designs. Recently, new bonded joint designs, such as the Pi-joint design, have been developed that minimize peel stresses and provide a more robust joint. The Pi-joint design includes a male and a female section bonded with an adhesive. This type of joint is more tolerant to damage and flaws than other joint designs. The Pi-joints are lighter and less expensive to fabricate than conventional mechanically fastened joints. Pi-joint designs may be used in metal-to-metal, composite-to-metal, or composite-to-composite joints. Even though the Pi-joint design has the potential to improve the performance of adhesive bonded joints, fabrication issues still exist.
Currently, methods that are used to adhesively bond the male and female section of a Pi-joint design include a plunge method, a tube withdrawal method, and an injection method.
The plunge method utilizes filling the female section of a Pi-joint design with an adhesive followed by plunging the male section of the Pi-joint design into the pool of adhesive. With the plunge method it is difficult to control the amount of pre-filled adhesive to avoid spill over, which creates a need for clean up of the adhesive. Furthermore, the male section of the Pi-joint design needs to be positioned correctly during plunging. If the male section is not at the proper depth, the adhesive must be removed and the process must be repeated. The fact that the adhesive starts curing immediately after being filled into the female section of the Pi-joint design may cause additional problems.
The tube withdrawal method utilizes a tube that is placed at the bottom of the female section of the Pi-joint design. The male section of the Pi-joint design is then positioned over the tube and all openings except the exit side where the tube is located are sealed with a tape. The adhesive is subsequently injected via the tube while the tube is being withdrawn from the Pi-joint design. After a certain time frame and prior to the curing of the adhesive, the tape must be removed and the adhesive is typically sealed with a sealant. The tube is usually withdrawn manually, which makes it difficult to maintain a certain constant rate. If the tube is withdrawn too fast, air pockets or voids may occur in the bonding area. If the tube is withdrawn too slow, the pressure of the adhesive may disbond the tape and the adhesive may blow-out and spill. In this case, a clean up of the spilled adhesive will be needed. Furthermore, the tape needs to be removed within a certain time frame after injection of the adhesive to avoid bonding of the tape with the adhesive. After removal of the tape, the adhesive typically needs to be sealed with a sealant.
The injection method includes positioning the male section of the Pi-joint design within the female section of the Pi-joint design and drilling holes into a leg of a Pi-leg of the female section of the Pi-joint design. The holes are used as ports to inject the adhesive. Before the adhesive can be injected, all openings are typically sealed with tape. The tape may disbond during injection when the pressure of the adhesive is too large and, in this case, the adhesive could blow-out and spill and, consequently, clean up of the adhesive would be required. As with the tube withdrawal method, the tape needs to be removed within a certain timeframe after injection of the adhesive. Also, the adhesive is typically sealed with a sealant after the removal of the tape. Furthermore, the bond length of the adhesive from the injection port is limited depending on the properties of the adhesive and the gap width to be filled. Typically, an injection port is needed about every 12 inches.
The difficulties associated with the above-mentioned methods for adhesively bonding the male and female section of a Pi-joint design could result in necessary repairs due to poor bonding. Furthermore, above-mentioned methods do not apply lean manufacturing concepts. Lean manufacturing technologies are aimed at eliminating “waste”, such as wasted material, wasted time, and wasted money. Lean technologies include simpler, cheaper, more tailored manufacturing systems that enable reduced cycle times and cost.
As can be seen, there is a need for a method to adhesively bond a male and a female section of a Pi-joint design that incorporates the principles of lean manufacturing concepts. Furthermore, there is a need for a method to adhesively bond a male and a female section of a Pi-joint design that improves the quality of the adhesive bond while reducing cycle times and cost, for example, by eliminating clean-up of spilled adhesive and by eliminating steps such as installation and removal of the tape to seal openings of the Pi-joint design.
There has, therefore, arisen a need to provide a process for adhesive bonding of the sections of a Pi-joint design that enables a strong adhesive bond between the male and female sections of a Pi-joint design while eliminating blow-outs of adhesive and, therefore, the need to clean-up spills. There has further arisen a need to provide a process for adhesive bonding that does not require sealing of a Pi-joint openings and the removal of tape after the injection of the adhesive. There has still further arisen a need to provide a process for adhesive bonding that enables longer injection bond length.